Experimental and Computational Investigation of the Tip Clearance Flow for an Axial Ventilation Fan

The tip leakage flow in an axial ventilation fan with various tip clearances is investigated by experimental measurement and numerical simulation. The characteristic of a ventilation fan is an extreme low-pressure difference, a large tip clearance with a low rotating speed. A three dimensional PDA (Particle Dynamics Analysis) system is used for the measurement of the velocity field in the tip clearance region. The flow field is surveyed across the whole passage at fifteen axial locations (from 100% axial chord in front of the leading edge to 100% axial chord behind the trailing edge), mainly focusing on areas close to the blade tip (from 90% of the blade span to the casing wall). Both experimental measurement and numerical simulation indicate that the leakage flow originating from the tip clearance along the chord rolls up into a three-dimensional spiral structure to form a leakage flow vortex. A low axial velocity zone shows up in the tip region, which leads to blockage of the main flow. There are under-turning zones near and in the blade tip region, and an overturning zone in a lower span region with a critical span-wise position of about 94%. A reverse flow appears at the suction side near the trailing edge. As the tip clearance increases, the tip leakage flow and the reverse flow become stronger and fully developed. In addition, the position of the first appearance of the tip leakage vortex moves further downstream in a direction parallel to the mid chord line.